1. Field of the Invention
The present invention relates to a thin-film solar cell and a manufacture method therefor, more particularly to a thin-film solar cell having a transparent electrode layer that allows improvement of performance of the thin-film solar cell including, for example, photovoltaic conversion efficiency, and a manufacture method therefor.
2. Description of Related Art
Solar cells are photovoltaic conversion devices which convert solar light into electric energy and are expected to provide clean energy alternative to nuclear energy and fossil energy such as petroleum. One of the most important challenges for putting solar cells into practical use is to reduce production costs.
The solar cells are roughly classified according to their materials into those using monocrystalline materials, those using polycrystalline materials, those using amorphous materials and those using thin films of compounds. Among them, those using amorphous materials and thin films of compounds are under active development because their materials are expected to allow the production costs to reduce greatly.
The production costs of solar cells should be calculated per unit electric power they produce, and it is considered that the production costs may be reduced by technological development concerning upsizing, high-speed and high-power operation of solar cells, etc.
As means for high-power operation, there is known a technique of forming a reflective film of a metal such as Al, Cu, Au or the like having a high reflectivity on a rear side, a side opposite to a light-incident side of a solar cell. This technique is intended to reflect light traveling through a semiconductor layer of a solar cell, which produces carriers, by the reflective layer to make the light absorbed again in the semiconductor layer, thereby increasing output current and improving photovoltaic conversion efficiency.
However, if a film of a reflective metal is used as a rear electrode, the metal itself diffuses into the semiconductor layer to deteriorate characteristics of the solar cell or to establish conduction with an electrode on the light-incident side and cause a shunt. That is marked particularly in the case where the rear electrode is formed to have a rough face for scattering light traveling through the semiconductor layer to increase the optical path of long wavelength light which has a small absorption coefficient.
On the other hand, a method is known which is intended to improve surface characteristics of the substrate, i.e., the flatness of the rear electrode and the adhesion of the semiconductor layer, or to prevent the metal of the rear electrode from forming an alloy with the semiconductor layer by intervention of a transparent electrode layer between the rear electrode and the semiconductor layer. It is also proposed that the method increases the sensitivity of the solar cell to spectra in a long wavelength region and reduces current running in defect regions of the semiconductor layer.
Thus the transparent electrode layer on the rear side of the thin-film solar cell has a significant role in improvement of the conversion efficiency. Typically, ZnO:Al is used in view of costs and stability of films formed thereof.
For forming the transparent electrode layer as a thin film with a large area, a magnetron sputtering apparatus is used, for example. Magnetron sputtering apparatus generally include (1) those of a magnet-fixed substrate-fixed type, (2) those of a magnet-moving substrate-fixed type and (3) those of a magnet-fixed substrate-moving type.
However, if (1) a magnetron sputtering apparatus of the magnet-fixed substrate-fixed type is used for forming a thin film on a fixed substrate with a large area, a large number of magnets are required to be mounted to cover a large film-formation region. Thus the apparatus becomes complicated and the production costs rise. Even if magnets are used in a great number, it is difficult to form a uniform magnetic field over the large film-formation region owing to non-uniform consumption of a target and other causes. Thus the film formed by deposition is still non-uniform.
In the case of (2) a magnetron sputtering apparatus of the magnet-moving substrate-fixed type, usually, a magnet moved reciprocally over the film-formation region many times. As compared with the apparatus (1), the film deposited on the substrate is more homogeneous and the target is used more efficiently. For example, Japanese Unexamined Patent Publication HEI 10(1998)-158833 describes that a magnet is moved reciprocally eleven times for forming an ITO (indium.tin oxide) film of 60 nm thickness. Japanese Unexamined Patent Publication 2001-32068 describes that a magnet is moved reciprocally two or more times for forming a single thin film of a metal oxide.
Yet neither the apparatus (1) nor (2) provides a sufficient fill factor as an output characteristic of solar cells owing to unsatisfactory quality of the obtained film as the rear transparent electrode layer and unsatisfactory junction state between the photovoltaic conversion layer and the transparent electrode layer. Further improvement in the photovoltaic conversion efficiency and reduction in the production costs are demanded.
Japanese Unexamined Patent Publication HEI 6(1994)-338623 discloses use of Ga-doped ZnO films for the rear transparent electrode layer instead of conventional Al-doped ZnO films.
However, a single film of Ga-doped ZnO does not contribute to reduction in the production costs. The change of a target with time prevents obtainment of stable film quality and declines the fill factor.
The present invention provides a thin-film solar cell comprising a transparent electrode layer; a semiconductor photovoltaic conversion layer; a rear transparent electrode layer; and a rear reflective metal layer, said layers being formed in this order on a transparent substrate, wherein the rear transparent electrode has a two-layer structure of an ITO or ZnO:Ga layer and a ZnO:Al layer formed in this order on the semiconductor photovoltaic conversion layer.
The present invention also provides a method for manufacturing a thin-film solar cell comprising forming a transparent electrode layer, a semiconductor photovoltaic conversion layer, a rear transparent electrode layer and a rear reflective metal layer in this order on a transparent substrate, wherein the rear transparent electrode layer is formed in a two-layer structure of an ITO or ZnO:Ga layer and a ZnO:Al layer which are continuously formed using a magnetron sputtering apparatus of a substrate-moving type having an ITO or ZnO:Ga target and a ZnO:Al target arranged in this order in a substrate-moving direction.
Thus, an object of the present invention is to provide a thin-film solar cell with a rear transparent electrode layer which can be manufactured by a high throughput production and exhibits a stable film quality and a good ohmic contact, thereby improving the output characteristic and further the photovoltaic conversion efficiency even of a thin-film solar cell with a large area. The invention also provides a manufacture method for the cell.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.